Concave holographic gratings are commonly used in tuneable monochromators for obtaining monochromatic light of one or several wave-lengths. A typical application of such a grating is the use in the optical unit of e.g. a chemical analysis instrument such as an HPLC (High Performance Liquid Chromatography) system wherein high intensity monochromatic light via an optical fiber impinges on a flow cell through which the liquid to be analyzed, usually containing a protein, is flowing. When mounting the concave holographic grating in the optical unit it is crucial that the grating is aligned in a correct position vis-à-vis an entrance port from which the light derives and an output port constituting the entrance of the optical fiber. In order to achieve a consistent output, the vertex of the grating should be held essentially at the center of revolving of the holder, i.e. to the center of the axis around which the grating is revolved with high precision when the grating is used to generate different wave-lengths. Previously this adjustment has been performed e.g. by a “tripod” arrangement with three non-orthogonal screws which implies that the adjustment is not pure vertical or pure horizontal but gives a tilt. Furthermore the adjustment implies that the position of the vertex of the concave surface of the grating may be moved away from the center of revolving of the holder. Thus there is a need for an improved holder for a concave holographic grating where the adjustment is less cumbersome.